Magnetic resonance imaging involving movement of patient&#39;s couch

ABSTRACT

A magnetic resonance imaging system performing various types of imaging that involves movement of a patient&#39;s couch. The system has a patient&#39;s couch having a tabletop movable in a predetermined direction passing through a static magnetic field as well as reception multiple RF coils consisting of for example a plurality of coil groups. The tabletop is automatically moved in its longitudinal direction in accordance with a length of each coil group in the predetermined direction. At each moved position, scanning is performed on a given pulse sequence. An echo signal is received through the multiple RF coils, then switched over by an input switchover unit to be sent to a receiving-system circuit. The echo signal is subjected to given processing in this circuit so that it is converted to echo data. The echo data are produced into an MR image by a host computer.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/075,269, filed Mar. 9, 2005, now U.S. Pat. No. 7,190,164; which is adivisional of U.S. patent application Ser. No. 09/841,171, filed Apr.25, 2001, now U.S. Pat. No. 6,946,836; which claims priority of JapanesePatent Application No. 124819/2000, filed Apr. 25, 2000. The entirecontents of these applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetic resonance imaging (MRI)system and a magnetic resonance imaging method, which are used formedical purposes, and in particular, to a system and method enablingfast and/or wide-range imaging with a couch (tabletop) on which apatient (object to be imaged) is laid.

2. Description of Related Art

Magnetic resonance imaging is a technique for magnetically excitingnuclear spins in an object located in a static magnetic field byapplying a radio-frequency (RF) signal with the Larmor frequency, andreconstructing an image of the object using an FID (free inductiondecay) signal or echo signal induced by the excitation. In the field ofmagnetic resonance imaging, like imaging using other modalities, a widevariety of imaging techniques have been developed thanks to recentadvancements in hardware.

For example, for imaging blood vessels in the inferior limb, variousimaging techniques are already known. Normally, imaging the inferiorlimb requires an imaged region to be wider in the body axis direction.Further, it is also required that imaging in such a wider range befinished during an interval of time during which a contrast agentremains in the inferior limb. Hence imaging should be done at a higherspeed (in a shorter time) and in a higher temporal resolution.

One technique, which is able to have a wider imaging range, is animaging technique in which the patient is moved on a couch, called amoving bed technique, by which the imaging is done with a patient'scouch moved. When this technique is used, the first scan for a givenregion is conducted at a certain patient's couch location, the couch(patient) is moved by a distance corresponding to the given region, andthen a second scan is conducted for the next region to be imaged. Thiscombination of scans and couch movements is repeated in turn, so that adesired region to be imaged, such as the inferior limb, is entirelycovered. After the imaging, the obtained images are lined up in, forexample, the body axis direction or combined into a single image to beused for diagnosis.

The moving bed technique includes a first technique that uses awhole-body coil with the couch moved and a second technique that usesmultiple RF coils.

In the case of the technique involving the whole-body coil, thewhole-body coil is fixed to a magnet. The couch is moved stepwise by adistance equal to a predetermined region to be imaged, and after eachmovement to a new region to be imaged, imaging is done.

FIG. 1 exemplifies a magnetic resonance imaging system capable ofperforming the moving bed technique by which a whole-body coil 101 isused. In the figure, reference 102 is a magnet that generates a staticmagnetic field. Connected to the whole-body coil 101 through a duplexer103 are a transmitter 104 and a preamplifier 105. The preamplifier 105is connected to a receiving-system circuit 106, both of which make up areceiver. A host computer 107 responsible for control of the wholesystem is placed to control a transmitter 104 and a gradient amplifier109 via a sequencer 108, so that a given pulse sequence is performed.Connected to the host computer 107 are an input device 109, display 110,and storage 111. The host computer 107 controls a couch driver(not-shown) to move a tabletop 112 of the couch based on the moving bedtechnique.

On the other hand, when the second technique is used with multiple RFcoils, the multiple RF coils themselves are fixed to an object or thecouch, so the coil can be moved with the couch. The multiple RF coilsconsist of, for example, a plurality of sets of coil members (a group ofcoils) disposed in an array. In this case, in response to movement ofthe couch under imaging, each set of coil members enters a uniformstatic field region within a magnet's bore, and then exists therefrom.

When it is desired to use the multiple RF coils based on the moving bedtechnique, it is conceivable that a plurality of images obtained througha plurality of sets of coil elements are made to cover all of a desiredregion to be imaged of an object.

FIG. 2 exemplifies a magnetic resonance imaging system that is able toexecute the moving bed technique with such multiple RF coils. Themultiple RF coils 121 consist of three sets of coil members (coilgroups) 1 to 3. Each set of the coil members 1 to 3 is independentlyrouted to a host computer 107 through a preamplifier 105 a (to 105 c)and a receiving-system circuit 106 a (to 106 c), respectively. That is,the multi RF coil 121 and the circuitry to receiver and process detectedsignals of the coil are added to the constitution shown in FIG. 1.

Meanwhile, to meet the foregoing requirements of shortening the imagingtime, higher-performance technologies in the hardware, as in improvementof a booster technology to shorten a switchover time of gradients, arenow under development.

As another imaging technique, high-temporal-resolution dynamic imagingis known for observing the passage of a contrast agent at high temporalresolution. This imaging includes a key-hole imaging technique (forexample, refer to “R. A. Jones at al., “Dynamic, contrast enhanced, NMRperfusion imaging of regional cerebral ischaemia in rats using k spacesubstitution” SMRM 1992, p.1138) and a view share technique (forexample, refer to U.S. Pat. No. 4,830,012). When the key-hole imagingtechnique is used, data for one image is acquired first, and thendynamic imaging is done, during which time only data mapped in a centralpart of the k-space used for image reconstruction are updated.Meanwhile, the view share technique, which abandons the procedures ofupdating all the k-space with new data before reconstructing the k-spaceinto an new image, uses the k-space previously divided in a plurality ofregions. An image is reconstructed whenever a plurality of dividedregions of the k-space are partly replaced with new acquired data, thusraising an image update rate. Another known imaging technique is called3D-TRICKS (for example, U.S. Pat. No. 5,713,358, which is provided byimproving the key-hole imaging technique to 3D MRA. The central part ofthe k-space is therefore raised in the data update rate with respect tothe remaining region, so that the key-hole imaging technique iseffective in observing the passage of a contrast agent at highertemporal resolution than that of the normal view share technique.

However, the above various types of conventional techniques have somedrawbacks. For using the moving bed technique that involves the multipleRF coils to widen a region to be imaged, it is required that a pluralityof sets of coil elements be switched over to select one set located atthe uniform static field region whenever the couch is moved by an amountcorresponding to the length of a region to be imaged. In order torealize this, as an operator observes the couch that is under stepwisemovement, the operator must manually switch over one set of coilelements to another set. This complicates operations and lacks accuracy,in addition to taking much time.

Further, the current hardware techniques are still lacking whenrealizing both of the shortened imaging time and the higher temporalresolution. The foregoing key-hole imaging and other imaging techniquesare still short of temporal resolution. In addition, the foregoingimaging techniques such as the key-hole imaging is simply raisingapparent temporal resolution by updating data mapped in part of thek-space, thus lacking a depiction performance of minute structures.Moreover, the foregoing imaging techniques such as the key-hole imagingcan be applied only to a situation that a region to be imaged of anobject is constant and changes of intensity at part of the region to beimaged, which is due to a contrast agent, are observed. But thosetechniques are unavailable for imaging that requires the couch to bemoved such that different regions are sequentially subject to imaging ata high speed.

Recently, concerning this fast imaging, a technique of gaining ashortened imaging time by using multiple RF coils has been spotlighted(for example, refer to “10^(th) Ann. Scientific Meeting SMRM 1240(1991),” which is effective in speeding up approximately all types ofmagnetic resonance imaging techniques. This technique enables imaging tobe performed with using less encoding steps than necessary forreconstructing a single image. The resultant folded data (i.e.,aliasing) is dissolved based on the fact that a plurality of coilelements constituting the multiple RF coils are different in theirsensitivity distributions from each other, so that images with no foldeddata are obtained. Since this fast imaging allows the number of encodingsteps to be lessened in proportion to the number of coil elements,unlike the ordinal imaging methods, the imaging time can be shortened.

However, the moving bed technique involving a single whole-body coilcannot be applied to such fast imaging. Further, in cases the moving bedtechnique involving multiple RF coils is applied to the fast imaging, itis necessary that an operator switch over a plurality of sets of coilelements, as described before. This means that the fast imaging cannotprovide its merits satisfactorily.

SUMMARY OF THE INVENTION

The present invention has been made to overcome the deficiencies of theforegoing imaging techniques. A first object of the present invention isto reduce operational work, speed up imaging, and increasing accuracy inmoving a patient's couch when multiple RF coils are used as a receptionRF coil so as to perform imaging based on the moving bed technique(method of moving the couch).

A second object of the present invention is to easily perform the fastimaging with improved temporal resolution, so that MR images of whichdepiction performance is improved are provided, in cases multiple RFcoil are used as a reception RF coil so as to perform imaging based onthe moving bed technique.

Still, a third object of the present invention is to provide fastimaging by moving the couch (tabletop) on which an object is laid, evenwhen only one reception RF coil is used.

Still further, a fourth object of the present invention is to performcontrast MR angiography at higher speed based on movement of the couch(tabletop) on which an object is laid, thus raising flexibility of themoving bed technique.

In order to accomplish the above objects, as one example of one aspectof the present invention, there is provided a magnetic resonance imagingsystem that comprises static magnetic field generating means forgenerating a static magnetic field containing a uniform region whosemagnetic intensity is uniform; a couch movable in a predetermineddirection passing through the static magnetic field, an object to beimaged being laid on the couch; a reception multiple RF coil including aplurality of coil members disposed toward the object; position changingmeans for automatically changing a relative position formed between thecouch and the static magnetic field generating means in thepredetermined direction in accordance with a length of each of theplurality of coil members detected in the predetermined direction;scanning means for scanning the object by applying a given train ofpulses to the object at each position changed by the position changingmeans; reception means for receiving through the multiple RF coils anecho signal that emanates responsively to the application of the trainof pulses by the scanning means; reception-processing means forprocessing, with given processing for reception, the echo signalreceived by the reception means so that the echo signal is convertedinto echo data; and image producing means for producing an MR imagebased on the echo data converted by the reception-processing means.

Preferably, the predetermined direction is a longitudinal direction ofthe couch and the position changing means is composed by means formoving a position of the couch in the longitudinal direction with thestatic magnetic field generating means being fixed. In such a case, theposition changing means is composed by means for changing the positionso that a center position of each of the plurality of coil members inthe longitudinal direction corresponds to the uniform region of thestatic magnetic field. Further, the reception processing means mayinclude selection means for automatically selecting, from the echosignals received individually by the plurality of coil elements, theecho signal received by a certain coil member located at the center ofthe uniform region in the longitudinal direction, the selected echosignal being given to the image producing means. For example, theselection means is able to include signal level detecting means fordetecting a level of the echo signal received by each of the pluralityof coil members, and signal selecting means for automatically selectingthe echo signal received by the coil member located at the center of theuniform region in the longitudinal direction on the basis of changes inthe level of the echo signal detected by the signal level detectingmeans.

Still preferably, the system may further comprise ID (identification)generating means for generating an ID number inherent to each coilmember, the ID producing means being disposed with each of the pluralityof coil members, size memorizing means for memorizing a size of each ofthe plurality of coil members in the longitudinal direction, the sizecorresponding to the ID number of each coil member generated by the IDgenerating means, disposal detecting means for identifying each signalline of the plurality of coil members so as to detect a disposal stateof the plurality of coil members in the longitudinal direction, anddetermination means for determining the size by making detectioninformation about the coil disposal state detected by the disposaldetecting means refer to the size memorizing means, the positionchanging means includes means for moving the position of the couch basedon the size determined by the determination means, and thereception-processing means includes selection means for automaticallyselecting, from the echo signal received by each of the plurality ofcoil members, a certain echo signal received by the coil member locatedat the center of the uniform region in the longitudinal direction on thebasis of the size determined by the determination means and the coildisposal state detected by the disposal detecting means, the detectedecho signal being given to the image producing means.

It is also preferred that the pulse sequence is set to include thenumber of encoding steps less than a given number of encoding stepsrequired for reconstructing the MR image by one, the position changingmeans is composed of means for changing the position so that, of theplurality of coil members constituting the multiple RF coils, anoverlapped region of sensitivity distribution regions of any two memberswhich are mutually-adjoining agrees with the uniform region of thestatic magnetic field in the longitudinal direction and moving the couchstep by step by a distance corresponding to each coil member in thelongitudinal direction, and the image producing means is composed ofmeans for performing unfolding processing on a set of the echo dataobtained by the reception processing means at every position of thecouch changed by the position changing means on the basis of differentsensitivity distributions of the plurality of coil members.

Preferably, each of the plurality of coil members constituting themultiple RF coils is an array type of RF coil has a plurality of coilelements.

Still, by way of example, each of the plurality of coil membersconstituting the multiple RF coils is a whole-body coil.

Further, the multiple RF coils may be fixed to one selected from a groupof the object and the couch.

According to another example of the magnetic resonance imaging systemaccording to the present invention, the system comprises static magneticfield generating means for generating a static magnetic field containinga uniform region whose magnetic intensity is uniform; a couch movable ina predetermined direction passing through the static magnetic field, anobject to be imaged being laid on the couch; at least a single receptionRF coil disposed fixedly to the static magnetic field generating means;position changing means for automatically changing a relative positionformed between the couch and the static magnetic field generating meansin the predetermined direction; scanning means for scanning the objectby applying a given train of pulses to the object at each positionchanged by the position changing means; reception means for receivingthrough the reception RF coil an echo signal that emanates responsivelyto the application of the train of pulses by the scanning means;reception-processing means for processing, with given processing forreception, the echo signal received by the reception means so that theecho signal is converted into echo data; and image producing means forproducing an MR image based on the echo data converted by thereception-processing means.

By way of example, the reception RF coil is one in number. For instance,the reception RF coil is a whole-body coil used in common fortransmission and reception. In such a case, the train of pulses is setto include the number of encoding steps less than a given number ofencoding steps required to reconstruct the MR image by one, and theimage producing means is composed of means for performing unfoldingprocessing on a set of the echo data obtained by the receptionprocessing means at every position of the couch changed by the positionchanging means on the basis of different sensitivity distributions ofthe plurality of coil members. As an example, the position changingmeans is composed of means for moving the couch every half of a lengthof the reception RF coil in the predetermined direction.

Still it is preferred that the position changing means may be composedof means for moving the couch to a first couch position and a secondcouch position, a region to be imaged of the object being located at thefirst couch position with the region shifted in part from a sensitivitydistribution region of the reception RF coil; and the region beinglocated at the second couch position with the region contained entirelyin the sensitivity distribution region of the reception RF coil, thesystem further including instruction means for instructing a contrastagent to be injected into the object when the couch is located at thesecond position. In such a case, as one example, the train of pulses isset to include the number of encoding steps less than a given number ofencoding steps required to reconstruct the MR image by one, the scanningmeans is composed of means for performing both a firstsensitivity-distribution measuring scan for measuring a sensitivitydistribution of the reception RF coil and a first imaging scan forobtaining the MR image of the region when the couch is located at thefirst couch position, and for performing both a secondsensitivity-distribution measuring scan for measuring a sensitivitydistribution of the reception RF coil and a plurality of times of secondimaging scans for obtaining the MR image of the region when the couch islocated at the second couch position, and the image producing meansincludes means for reconstructing the echo data obtained by both of thefirst and second imaging scans into image data and means for unfoldingthe image data obtained through each of the second imaging scans byusing both of the echo data obtained through the first and secondsensitivity-distribution measuring scans and the image data obtainedthrough the first imaging scan.

Still, for example, the reception RF coil is one in number.

According to a further aspect of the present invention, there isprovided an MR imaging method of obtaining an image of an object basedon a sub-encoding technique (fast imaging technique) using a receptionRF coil, the object being laid on a couch, the method comprising thesteps of: acquiring by acquiring means data of coil sensitivitydistributions of the reception RF coil and image data at a plurality ofpositional relationships between a region to be imaged of the object andthe reception RF coil; and unfolding by data processing means the imagedata acquired at each position of the object using the data of the coilsensitivity distributions.

The remaining configurations and features of the present invention willbe described in the embodiments and with appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a block diagram of a conventional magnetic resonance imagingsystem for imaging carried out using a whole-body coil on a moving bedtechnique;

FIG. 2 is a block diagram of a conventional magnetic resonance imagingsystem for imaging carried out using multiple RF coils on the moving bedtechnique;

FIG. 3 shows a block diagram outlining the configuration of a magneticresonance imaging system according to a first embodiment of the presentinvention, the system being directed to imaging using multiple RF coilson the moving bed technique;

FIG. 4 is a flowchart outlining procedures of imaging performed in thefirst embodiment on the moving bed technique;

FIG. 5 shows a block diagram outlining the configuration of a magneticresonance imaging system according to a second embodiment of the presentinvention, the system performing imaging using multiple RF coils on themoving bed technique;

FIG. 6 shows a block diagram outlining part of the configuration of amagnetic resonance imaging system according to a third embodiment of thepresent invention, the system performing imaging using multiple RF coilson the moving bed technique;

FIG. 7 illustrates the relationship between the switch positions of anID generator and pieces of coil information;

FIG. 8 is a flowchart outlining tabletop movement control based oninformation given by the ID generator;

FIG. 9 illustrates markers used for recognizing the position of a coilelement, which shows a modification;

FIG. 10 is an outlined schematic diagram showing part of a magneticresonance imaging system according to a fourth embodiment of the presentinvention, the system performing fast imaging using multiple RF coilswith a couch moved;

FIGS. 11A and 11B are outlined schematic diagrams each showing part of amagnetic resonance imaging system according to a fifth embodiment of thepresent invention, the system performing fast imaging using multiple RFcoils with a couch moved;

FIG. 12 is an outlined schematic diagram showing part of a magneticresonance imaging system according to a sixth embodiment of the presentinvention, the system performing fast imaging using multiple whole-bodycoils with a couch moved;

FIGS. 13A and 13B illustrate the configuration of multiple whole-bodycoils according to a modification;

FIG. 14 is an illustration of coil positions and a region to be imagedrealized by fast imaging according to a seventh embodiment of thepresent invention, the imaging being performed using a single whole-bodycoil with a couch moved;

FIG. 15 outlines the configuration of a magnetic resonance imagingsystem according to an eighth embodiment of the present invention, thesystem being directed to fast contrast MRA using a single reception coilwith a couch moved;

FIGS. 16A and 16B illustrate the positional relationship between movedpositions of a couch's tabletop (a first and second couch's tabletoppositions) and the sensitivity region of the reception coil;

FIG. 17 is a flowchart outlining processing carried out by a hostcomputer during the fast contrast MRA; and

FIG. 18 illustrates timing among scans for the fast contrast MRA, couchmovement, and injection of a contrast agent, and acquisition of dataused for unfolding processing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will now be described withreference to accompanied drawings.

Referring to FIGS. 3 and 4, a magnetic resonance imaging (MRI) system ofa first embodiment will now be described.

The magnetic resonance imaging system performs imaging using multiple RFcoils with a moving couch (that is, a moving bed technique).

The configuration of this magnetic resonance imaging system is outlinedin FIG. 3. This system has a couch unit on which a patient P as anobject to be imaged, a static magnetic field generating unit forgenerating a static magnetic field, and a gradient generating unit foradding position information to the static magnetic field. The systemalso has a transmission/reception unit for transmitting and reception RFsignals and a control/calculation unit in charge of control of theentire system and image reconstruction.

The static magnetic generation unit is proved with a magnet 1constructed by employing for example a superconducting magnet and astatic power supply 2 supplying current to the magnet 1. By thesedevices, a static magnetic field H₀ is produced in an axial direction ofa cylindrical bore (a space for diagnosis) into which the object P isinserted. In the orthogonal coordinate axes assigned to the system, suchdirection agrees with the Z-axis direction. In addition, this generationunit is provided with a shimming coil (not-shown) to which current issupplied from a shimming coil power supply to shim the static magneticfield. This supply of current forms a region in which the staticmagnetic field is kept to in magnitude within a certain range ofuniformity, that is, a region for diagnosis.

The couch unit is configured in such a manner that a tabletop 14T onwhich the object P is laid can be inserted and returned into and fromthe bore of the magnet 1. A couch driver 14D does this insertion andreturn. The couch driver 14D responds to a drive signal given by a hostcomputer 6 later described so as to move the tabletop 14T along itslongitudinal direction (i.e., the Z-axis direction). The object P islaid on the tabletop 14T along its longitudinal direction, for example.

The gradient generating unit has a gradient coil unit (not-shown)incorporated in the magnet 1. The gradient coil unit includes three sets(types) of x-, y-, and z-coil for generating X-, Y-, and Z-directionalgradient magnetic fields (gradients) orthogonal among themselves. Thegradient generating unit further includes a gradient amplifier 4 tosupply currents to each of the x-, y-, and z-coils. The amplifier 4supplies those coils with pulsed currents to generate gradients underthe control of a sequencer 5 later described.

Controlling pulsed currents supplied from the gradient amplifier 4 tothe x-, y-, and z-coils enables the mutually-orthogonal X-, Y-, andZ-directional gradients, which are gradients in the physical axes, to besynthesized. Thus mutually-orthogonal logic directional gradients, i.e.,a slice gradient Gs, phase-encode gradient Ge, and read-out (frequencyencoding) gradient Gr can be set or changed in an arbitrary way. Thegradients in the slice, phase-encode, and read-out directions aresuperposed on the static field H₀.

The transmission/reception unit includes a whole-body (WB) coil 7T andmultiple RF coils 7R, both of which are RF coils, and a transmitter 8Tand a receiver 8R both connected to the coils 7T and 7R. The coils 7Tand 7R are disposed in the vicinity of the object P in the space fordiagnosis formed within the bore of the magnet 1.

The whole-body coil 7T is used in common for transmission and receptionin cases when only the coil 7T is employed as the RF coil. By contrast,when employing the multiple RF coils 7R (serving as a reception coil),the whole-body coil 7T is used as a transmission coil.

The multiple RF coils 7R, formed as an array type of coil to which ahigh S/N (signal to noise) ratio can be given, are disposed in turn inthe region in which the static field is uniform (region for diagnosis),so that imaging can be done using the moving bed technique. The multipleRF coils 7R are formed by at least one array coil (each array oilconsists of a plurality of coil elements, which are called “coil group”in this embodiment). Each array coil constitutes the coil memberaccording to the present invention. By way of example, the multiple RFcoils 7R according to the present embodiment are formed three coilgroups 1 to 3 disposed on the tabletop 14T of the couch. Each coil group1 (to 3) is an array type of coil in which a plurality of coil elementsare arranged. The coil groups 1 to 3 can be mounted on the object. Theoutput lines of the three groups 1 to 3 are, separately from each otherand coil element by coil element, connected to the host computer 6. Thusthe output signal from each coil element is supplied to the hostcomputer 6 independently of each other.

Both the transmitter 8T and the receiver 8R are operative under thecontrol of a sequencer 5 later described. The transmitter 8T supplies tothe whole-body coil 7T RF pulsed currents of which frequency is set to aLarmor frequency to cause nuclear magnetic resonance (NMR) at magneticspins of the object P. On the other hand, the receiver 8R accepts anecho signal (RF signal) received by the whole-body coil 7T or multipleRF coils 7R, then makes it into echo data (i.e., raw data).

Specifically, the receiver 8R is divided, as shown in FIG. 3, into areception part for the whole-body coil and a further reception part forthe multiple RF coils.

The reception part for the whole-body coil includes a duplexer 81connected with the whole-body coil 7T, a preamplifier 82 connected tothe duplexer 81, and a receiving-system circuit 83 receiving a receptionsignal from the preamplifier 82. The duplexer 81 is coupled with thetransmitter 8T as well.

This connection permits the duplexer 81 to pass a transmission drivepulse from the transmitter 8T to the whole-body coil 7T in transmission,while to pass an echo signal detected by the whole-body coil 7T to thepreamplifier 82 in reception. The preamplifier 82 pre-amplifies thereceived echo signal to send the amplified signal to thereceiving-system circuit 83. This circuit 83 performs various types ofsignal processing on the inputted echo signal, the processing includingintermediate frequency conversion, phase detection, low-frequencyamplification, and filtering. Then, the processed signal is subject toA/D conversion to produce echo data (raw data), before being sent to thehost computer 6.

On the other hand, the reception part for the multiple RF coils isprovided with preamplifier groups 84A to 84C receiving echo signals formthe multiple RF coils 7R every coil group 1 (to 3) and every coilelement. The output lines from each preamplifier group 84A (to 84C) arerouted to an input switchover unit 86 via each connector box 85A (to85C) to which the lines can be connected detachably. Connected positions(or a connection order) of the preamplifier groups 84A to 84C to theconnector box 85A to 85C are determined in advance.

The input switchover unit 86 is for example composed of a multiplexerand switched over in response to a switchover control signal SS that thehost computer 6 gives. Accordingly, the input switchover unit 86 is ableto selectively switch over any one of the signals that the preamplifiers84A to 84C produce and to send out the selected signal through itsoutput terminal. The output terminal of the input switchover unit 86 isthen sent to the host computer 6 via a further receiving-system circuit87. In the similar way to the above, this circuit 87 executes varioustypes of processing, such as intermediate frequency conversion, phasedetection, low-frequency amplification, and filtering, on the inputtedecho signal. Further, the circuit 87 A/D-converts the processed echosignal to produce echo data, then sends the converted one to the hostcomputer 6.

Moreover, the control/calculation unit includes a sequencer 5 (alsocalled a sequence controller), host computer 6, storage 11, display 12,and input device 13.

The host computer 6 provides the sequencer 5 with pulse sequenceinformation on the basis of procedures on software stored in its innermemory or the storage 11 and controls the operations of the entiresystem. Additionally, the host computer 6 has various functions ofcalculating image data through reconstruction processing of echo dataand controlling the drive of the couch driver 14D. The host computer 6further includes a function of selectively switching over the inputswitchover unit 86 by giving a switchover control signal SS theretoevery time when, of a plurality of times of imaging, one imaging areswitched over to another. This switchover function is previouslydetermined by the software procedures so that the input/output paths ofthe input switchover unit 86 are switched over in a predetermined order.

The imaging scan is carried out on the imaging pulse information inorder to acquire a set of echo data necessary for image reconstruction.The pulse sequence uses either a three-dimensional (3D) scan or atwo-dimensional (2D) scan. As modes of trains of pulses which can beemployed, provided are an SE (spin echo) method, FSE (fast spin echo)method, FASE (fast asymmetric SE) method (that is, the FSE methodcombined with a half-Fourier method), EPI (echo planar imaging) imaging,FE (gradient field echo) method, FFE (fast FE) method, segmented FFEmethod, and others.

The sequencer 5 has a CPU and memories and memorizes pulse sequenceinformation sent from the host computer 6. Based on this information,the sequencer 5 controls the operations of the gradient amplifier 4,transmitter 8T, and receiver 8R. The pulse sequence information is allkinds of information necessary for operating the gradient amplifier 4,transmitter 8T, and receiver 8R according to a series of pulse sequence.For instance, the pulse sequence information includes pieces ofinformation concerning magnitudes of pulsed currents applied to the x-,y-, and z-coil, duration of application of the pulses, and applicationtiming.

Echo data (original data or raw data) processed by the receiver 8R aresent to the host computer 6, which has a function of imagereconstruction on the basis of a predetermined calculation program.Therefore, the host computer 6 uses its calculation function to map echodata given by the receiver 8R in a Fourier space (referred to as ak-space or frequency space) formed by its inner memory. The hostcomputer 6 carries out a two- or three-dimensional Fourier transform oneach set of echo data, so that image data in the real space arereconstructed. The image data are then subjected to a display actionconducted by the display 12 as well as stored by the storage 11. Imagingconditions, the type of a pulse sequence, and information with regard toimage synthesis or subtraction are provided to the host computer 6 viathe input put 13 depending on operator's desire.

Though not shown, the storage 11 has a memory (serving as a recordingmedium) in which stored is a program for MR imaging based on the fastimaging technique involving both the moving bed technique and couchmovement, which are derived from the present invention. The hostcomputer 6 reads in the program when the system is activated, and isused to perform the MR imaging.

In this magnetic resonance imaging, imaging that uses the moving bedtechnique is carried out, as shown in FIG. 4. The host computer 6commands the processing shown in FIG. 4.

At first, the tabletop 14T is moved such that the center of the firstcoil group 1 in the Z-axial direction agrees with the center of theuniform region of the static field in the Z-axial direction (Step S11 inFIG. 4). Responsively to this location, the host computer 6 provides theinput switchover unit 86 with the switchover control signal SS, so thatits input-side switchover terminal is switched over to “1,” that is, theside of the connector box 85A (i.e., the preamplifier 84A) (Step S12 inFIG. 14).

At this tabletop position and this switch's switchover position, thefirst scan is performed based on pulse sequence information given by thesequencer 5 (Step S13 in FIG. 4). This scan allows all echo signalsacquired from the all the coil groups 1 to 3 via the multiple RF coils7R to be sent to the receiver 8R. However, since the input path of theinput switchover unit 86 in the receiver 8R is switched over to only theside of the coil group 1, an echo signal acquired by the coil group 1 issolely sent to the receiving-system circuit 87.

Thus the echo signal detected by the coil group 1 is received andprocessed by the receiving-system circuit 87, with the result that theecho signal is converted to echo data, then sent to the host computer 6.In other words, of the echo signals detected by the all the coil groups1 to 3, only an echo signal emanated from any coil group present in theuniform region of the static field is processed and sent to the hostcomputer 6 (Step S14 in FIG. 4).

The host computer 6 performs reconstruction processing of echo datacorresponding to the echo signal detected by the coil group 1 (Step S15in FIG. 4). Real-space image data of a slice or slab of the object P arethus obtained, and temporarily stored in its inner memory of the hostcomputer 6 or the storage 11.

Then, in order to cope with the second scan, the tabletop 14T of thecouch (i.e., the object P) is moved by a distance corresponding to aregion required for one time of imaging in the Z-axis direction (StepS16 in FIG. 4). Thus the second coil group 2 is located in the centralpart of the uniform region of the static field in the Z-axis direction.Namely, in cases all the coil groups 1 to 3 is the same in size, anamount of movement of the tabletop in the Z-axis direction is the samewhenever each time of imaging is carried out. Responsively to thispositioning, the host computer 6 sends the switchover control signal SSto the input switchover unit 86 in order to switch over its input-sideswitchover terminal to “2,” i.e., the side of the connector box 85B(corresponding to the preamplifier group 84B) (Step S12 in FIG. 4). Atthis positioning and under this input switchover state, the second scanis carried out in the same way as the above (Step S13 in FIG. 4). As aresult, only an echo signal detected by the second coil group 2 isreceived and processed by the receiving-system circuit 87, the resultantecho data being given to the host computer 6. By this host computer 6,the echo data are reconstructed to image data (Steps S13 to S15 in FIG.4).

In the similar manner to the above, the tabletop 14T is further moved inthe Z-axis direction by a distance corresponding to the region requiredfor one time of imaging. In parallel with this, the input-sideswitchover terminal of the input switchover unit 86 is switched over“3,” (that is, the preamplifier 84C). In this situation, the third scanis carried out in the similar manner to the above (Steps S16, S17, S12and S13 in FIG. 4). Accordingly, only an echo signal detected by thethird coil group 3 is received and processed by the receiving-systemcircuit 87, thereby resultant echo data being sent to the host computer6. The echo data are reconstructed into image data by the host computer6 (Steps S14 and S15 in FIG. 4).

In this way, the multiple RF coil 7R is used as a reception RF coil toperform each time of scan based on the moving bed technique. After theacquisition of echo data, the host computer 6 synthesizes a plurality ofimages that have been reconstructed in response to echo signal detectionusing each coil group 1 (to 3) into an entire image with their positionsin the X-axis direction made to agree with each other. This produces,for example, a coronal image that covers the entire inferior limb.

Therefore, in the magnetic resonance imaging system according to thepresent invention, the single receiving-system circuit is enough evenwhen a plurality of coil groups 1 to 3 constituting the multiple RF coil7R. It is therefore possible to reduce the number of receiving-systemcircuits that conventional. Particularly, the capacity of a memorybuffer incorporated in the receiving-system circuit can largely belowered in number. Further, because it is not necessary for an operatorto manually set a tabletop's position with the position observed by theoperator, the operation is simplified greatly and the tabletop can belocated at a desired position with precision. In addition, this way ofautomatic tabletop movement enables the imaging to speed up.

On the contrary, in the conventional construction shown in FIG. 2, allecho signals detected by all the coil groups of the multiple RF coil aresent to the host computer via the individual receiving-system circuits.This configuration will still do if the number of coil groups is less,but if each coil group adopts a large number of coil elements, thenumber of receiving-system circuits increases correspondingly to that ofcoil elements. The capacities of buffer memories temporarily storingA/D-converted echo data should be extremely large. For instance, ifthree coil groups are used, each coil group (array coil) consisting offour-channel array types of coil elements, a total of 12-channnelreceiving-system circuits are required. By contrast, it is enough forthe receiver 8R according to the present embodiment to use only onereceiving-system circuit, expecting that preamplifiers are prepared foreach coil element of each coil group and one input switchover unit isprepared. The largeness of the circuit is lowered to a grater degree.

Referring to FIG. 5, another embodiment of the present invention willnow be explained. This embodiment includes the feature that theforegoing input switchover unit 86 operates to switch over according tolevels of echo signals.

In a magnetic resonance imaging system shown in FIG. 5, the receptionpart for the multiple RF coil in the receiver 8R employs signal leveldetectors 88A to 88C. Each of the detectors is inserted, coil group bycoil group, between each of the connector boxes 85A to 85C and each thepreamplifier groups 84A to 84C. Each signal level detector 88A (to 88C)detects levels (amounts of power) of echo signals emanated from eachcoil group 1 (to 3). Detected signals SC from those detectors 88A to 88Care sent to the host computer 6.

The multiple RF coils 7R are entirely moved step by step along theZ-axis direction every each time of imaging. Accordingly, the coilgroups 1 to 3 enters the uniform region of the static field in a certaindirection in turn and go out of the region in the opposite direction inturn. Hence an echo signal from a certain coil group existing in theuniform region is higher in intensity than that from the remaining coilgroups that are not present in the uniform region. The host computer 6accepts the detected signals SC form the signal level detectors 88A to88C to determine their signal levels. Specifically, a certain coil groupthat provides the highest signal level is selected. The host computer 6then provides the input switchover unit 86 with the switchover controlsignal SS so as to switch over the input switchover unit 86 in such amanner that the echo signal from the selected coil group is sent to thereceiving-system circuit 87.

The remaining configuration and imaging manners using the moving bedtechnique are identical to those in the first embodiment.

As described above, the reception path is automatically switched overbased on the levels of the signals from the coil groups 1 to 3.Accordingly, like the foregoing embodiment, the single receiving-systemcircuit is enough to the multiple RF coil 7R. The configuration of thesystem can be simplified and compacted to a larger extent.

It is unnecessary to set an amount of movement of the couch for everytime of imaging. Also it is unnecessary to know information about wheneach coil group was present in the uniform region of the static field(the region for diagnosis). Even when the lengths of the coil groups 1to 3 in the Z-axis direction are different from each other and/or thecoil groups are disposed to be oblique to the Z-axis direction, suchautomatic switchovers can be done, thus substantially saving labor foroperations.

In addition, in the magnetic resonance imaging system of thisembodiment, an alternative control is possible. That is, the movement ofthe tabletop 14T can be controlled using echo signal levels detectedwhen each coil group 1 (to 3) existed in the uniform region of thestatic field with the Z-axis directional center of each coil groupplaced at the center of the region. Practically, as the sequencer 5executes a positioning scan periodically, the host computer 6 controlsthe Z-axis directional movement of the tabletop 14T. Until the detectedsignals from the signal level detectors 88A to 88C reach a predeterminedsignal level, the tabletop 14T (that is, the multiple RF coils 7R),which is located at its predetermined initial position, is moved everytime of imaging. This allows the tabletop 14T to be moved in anautomatic function.

This automation makes it possible that the tabletop 14T is moved in acontrolled manner independently of the Z-axis directional sizes of thecoil groups or arranged situations of the coil groups. For example, suchcases include a situation that the coil groups 1 to 3 that compose themultiple RF coil 7R are made into different sizes that agree with eachpart of an object, respectively, thus the Z-axis directional sizes ofthe coil groups being different from each other. Further included is asituation that the Z-axis directional sizes of all the coil groups areequal to each other, but each coil is not necessarily disposed inposition in the Z-axis direction. That is, even if the coil groups aremutually different in the Z-axis directional substantial or apparentsizes, each coil group can be located in turn in the center of theuniform static field region in the Z-axis direction every time ofimaging. This automatic control of the tabletop reduces operator'soperations drastically.

Referring to FIGS. 6 to 8, a further embodiment of the present inventionwill now be described. This embodiment relates to another example todetect the positions of coil groups that composes a multiple RF coil.

FIG. 6 shows an essential part of a receiver 8R of a magnetic resonanceimaging system according to this embodiment. As shown therein, an IDgenerators 7A to 7C are placed in a plurality of coil groups 1 to 3 thatcomposes a multiple RF coil 7R, respectively. Each of the ID generators7A to 7C is for example a two-bit dip switch. The dip switch is notalways limited to 2 bits. As shown in FIG. 7, each dip switch generatesidentifying information indicative of the type and coil size of eachcoil group depending on its switched positions. The type of coil groupincludes a surface coil, QD coil, while the size of coil group includesthe largeness of length in the Z-axis direction. The relationshipsbetween the switched positions and the identifying information for eachcoil group are previously stored as tables in an inner memory of thehost computer 6.

As each coil group (consisting of a plurality of coil elements disposedin an array), coil elements of a desired type and a size are selectedappropriately, then disposed along the Z-axis direction, that is, alongan object P or the tabletop 14T. A plurality of disposed coil groups 1to 3 compose a multiple RF coil 7R. For instance, if a region to beimaged is the inferior limb, the coil groups of appropriate types andsizes are selected according to the shape and size of each part of theinferior limb. It is not absolutely necessary to dispose the coil groupsalong the geometrical Z-axis. By way of example, a certain coil groupmay be disposed along the femur part, thus slight oblique to the Z-axisdirection.

The plurality of coil groups 1 to 3 disposed as above are connected toslots 1 to 3 of a connector box 85 in the order of the disposition. Forexample, the wires W1 of the first coil group 1 which is to be locatednearest to the patient's head are connected to the first slot 1, thewires W1 including a signal line from the ID generator 7A and signallines from the coil elements. The second and third coil groups 2 and 3are connected in the similar way, so that their wires W2 and W3 areconnected to the slots 2 and 3 in turn. In this way, the coil groups 1to 3 can simply be connected to the slots 1 to 3 of the connector box 85in the disposed order of the coil groups, independently of types andsizes of the coil groups 1 to 3.

Of the output signal terminals of the connector box 85, the signal linesresponsible for outputs of the coil groups 1 to 3 are routed to thereceiving-system circuit 87 via the input switchover unit 86, as shownin FIG. 5. Meanwhile, the signal lines responsible for outputs of the IDgenerators 7A to 7C are coupled with the host computer 6.

The host computer 6 executes control represented in FIG. 8 using theswitch signals from the ID generators 7A to 7C, in other words,identifying information of the coil groups. Specifically, the hostcomputer 6 reads the switch signals, then makes reference to thepreviously-stored table exemplified in FIG. 7 to determine a disposalorder of the coil groups 1 to 3 and the type of each coil group (Step S1and S2). The host computer 6 obtains the sizes of the coil groupsthrough, for example, making reference to the table and amounts ofmovement of the coil groups 1 to 3 that should be moved for each time ofimaging are calculated based on the obtained sizes (Step S3).

On completion of this preparation, the host computer 6 begins to waitfor the next action as it determines whether timing for imaging has comeor not (Step S4). During the waiting, if imaging timing is detected, thehost computer controls a not-shown couch driver so that the tabletop 14Tis moved by a distance corresponding to the first movement amount (StepS5). Then, it is determined whether or not all the imaging has completedor not, and if imaging remains, the processing at Steps S4 and S5 willbe repeated (Step S6). This repetition leads to the second and thirdimaging that also involves the movement of the multiple RF coil 7R (thatis, the coil groups 1 to 3). After the completion of all the times ofimaging, the tabletop 14T is controlled such that it returns to itsinitial position predetermined in the Z-axis direction (Step S7).

In this embodiment, it is therefore unnecessary for an operator to pay aparticular attention to the relationship between types and sizes of thecoil groups to be used and movement amounts on the moving bed technique.Identifying information provided by the ID generators 7A to 7C can beused to automatically calculate movement amounts of the tabletop 14T.Thus, the tabletop movement is automatically controlled, during whichtime the input switchover unit 86 is automatically switched over inresponse to each time of imaging. Accordingly, an echo signal detectedby only a certain coil group 1 (to 3) located in the uniform staticfield region is received for imaging.

Based on the moving bed technique involving the multiple RF coil 7R, anMR image can therefore be obtained over a wired area. Concurrently, agreat deal of labor can be saved for operator's operations.

A modified example from the above will now be described with referenceto FIG. 9. This example, applicable to the foregoing embodiment, relatesto a technique of determining accurate amounts of movement of thetabletop (that is, the multiple RF coil) from positioning informationtoward the first coil group.

A marker MK showing the central position of each coil group 1 (to 3) inthe Z-axis direction is put on the coil groups. At first, a lightprojector is spotlighted onto the marker MK to assign the centralposition of each coil group to predetermined positions of a couch'spositional encoder. Then the light projector is used to locate itsspotlighted position Pz to a desired region to be diagnosed of apatient, before the position Pz is memorized. A coil size Ln is foundfrom the identifying information of each coil group. Subtracting alength A between the spotlighted position Pz and the coil center fromhalf of the coil size Ln gives the residual length from the currentposition Pz to the first coil group with precision. Accordingly, thecoil groups can be disposed at any position on the couch. Even when thecentral position of each coil group is not always consistent with anactually spotlighted position, the tabletop can be moved with precision.

A further modification will now be described, which is also applicableto the foregoing embodiment. This modification provides anotherconfiguration to detect the position of each coil group. To be specific,a minute pickup coil is disposed at the center of each of a plurality ofcoil groups. With the coil groups shifted by a predetermined distancefrom the center of the uniform static field region, a measuring gradientpulse of a predetermined magnitude is given to the coil groups. Thispreviously provides the relationships between the known shifted distanceand output signal values of the pickup coils. The output signal valuescan be given as waveform height values of output waveforms calculatedusing an integrator. In imaging, in cases the positions of the coilgroups are desired, the measuring gradient pulse is applied to thepickup coils to detect output signals therefrom, thus providing theZ-axis directional positions of the coil groups. Imaging on the movingbed technique with the pickup coils requires that the pickup coils beable to detect the Z-axis direction gradient pulse.

Further, where the multiple RF coil is made up of a plurality of coilgroups, such as surface coils, to which attention must be paid withregard to problem of a spatial disposal, a pickup coil for theorthogonal three directions is attached to a support of each coil group.In this case, the measuring gradient pulse is applied in sequence in thethree directions. Using the previously obtained positional relationshipbetween each coil center and its support, output signals of the pickupcoils are corrected.

Referring to FIG. 10, another embodiment of the present invention willnow be described. This embodiment concerns fast imaging using a multipleRF coil on the basis of the moving bed technique.

FIG. 10 pictorially shows the positional relationship between a multipleRF coil 7R and sensitivity regions of coil groups thereof in a magneticresonance imaging system according to the present embodiment. As shown,the coil groups 1 to 3 of the multiple RF coil 7R are disposed along theZ-axis direction. Responsively to the movement of the tabletop 14T, thethree coil groups 1 to 3 are also moved in the Z-axis direction.

Echo signals detected by the coil groups 1 to 3 are sent to a receiverof which circuitry is constructed in the same way as that in any of thefirst to third embodiments, in which the signals are processed by any ofthe foregoing processing ways. In this case, the foregoing inputswitchover unit is designed so that it allows two adjoining coil groupsto simultaneously be coupled with the receiving-system circuit. Thereceiving-system circuit includes a set of circuits to process, coilelement by coil element, the two echo signals emanated from the two coilgroups. This makes it possible that the two echo signals are receivedand processed in parallel, before being sent, signal by signal, to ahost computer as echo data.

Commands issued from the host computer or sequencer permits the tabletop14T to move along the Z-axis direction. Each of the coil groups 1 to 3is located in the uniform static field region (the region for diagnosis)every time of imaging, but in the present embodiment, there is no needto locate the center of a sensitivity region R_(sens) of each coil groupat the center of the uniform static field region (not shown),differently from the ordinal moving bed technique. Instead, a regionR_(overlap) in which sensitivity distribution regions R_(sens) of twoadjoining coil groups are mutually overlapped is set in the uniformstatic field region in order to perform imaging.

One time of scan is carried out with half of the number of steps ofencoding necessary for reconstructing a single image. When each time ofscan is ended, the couch is moved for the next scan by a distance forone time of scan, the distance being able to be scanned by one coilgroup. This combination of the scan and the couch movement is repeatedto scan an entire region to be desired.

More practically, this imaging can be carried out based on aconventional known technique of fast imaging using a multiple RF coil.Such technique has been provided by, for example, “10^(th), Ann.Scientific Meeting SMRM. 1240, 1991.” Reducing the number of encodingsteps than that necessary for a signal image will result in an imagewith aliasing. But the above processing technique includes calculationto remove the aliasing. In other words, the fact that the coilsensitivity distributions of the coil groups are different from eachother is utilized to decompose the aliasing through post-calculation,then the aliasing is removed from the image.

Accordingly, the present embodiment enables the fast imaging to beperformed using the moving bed technique. In particular, through themultiple RF coil is used, a plurality of sets of coil elements thatcomposes the multiple RF coil are automatically moved (tabletopmovement) and positioned, in addition to automatic switchovers betweenthe coil sets. Therefore, all the advantages of the fast imaging can beprovided, with fewer operators burden on operations.

In cases, as described above, the fast imaging with the multiple RF coilis conducted under the moving bed technique, the number of encodingsteps can be lowered in inverse proportion to the number of coil groups(elements). Thus, imaging time is reduced and a fully high speed ofimaging is maintained.

Referring to FIGS. 11A and 11B, another embodiment of the presentinvention will now be described. This embodiment relates to the disposalof a multiple RF coil.

As shown in FIGS. 11A and 11B, a multiple RF coil of this embodiment isdisposed as far apart as possible from the tabletop 14T of a couch orthe surface of an object P. Specifically, the multiple RF coil 7Rconsists of three coil groups 1 to 3. Each coil group 1 (to 3) consistsof an array type of upper coil group 1 (to 3) and an array type of lowercoil group 1 (to 3). The upper coil group 1 (to 3) has two coil elements7U₁ and 7U₂ located above the upper side of the tabletop and supportedby a common coil supporter 30U. The lower coil group 1 (to 3) has twocoil elements 7L₁ and 7L₂ located below the lower side of the tabletopand supported by a common coil supporter 30L.

Compared to the multiple RF coil shown in FIG. 10, this disposal widensthe sensitivity distribution region R_(sens) of each coil group, thuswidening an overlapped region R_(overlap) of the sensitivitydistribution regions between adjoining coil groups. If a region to beimaged is the same in area, the above disposal allows imaging to beaccomplished with a less number of coil groups.

Referring to FIG. 12, another embodiment of the present invention willnow be described. This embodiment is concerned with a configuration thatemploys a whole-body (WB) coil as multiple RF coils.

As shown FIG. 12, there is a whole-body coil 7 composed of an array ofmultiple coils, which are configured by placing three short-axiswhole-body coils 7 a, 7 b and 7 c in an array. The number of disposedcoils is at least two. The whole-body coils 7 a, 7 b and 7 c compose thecoil members of the present invention. The respective whole-body coils 7a, 7 b and 7 c are routed, in a receiver 8R, to respective preamplifiers32 a, 32 b and 32 c via a duplexer 31. The preamplifiers 32 a, 32 b and32 c are connected to a host computer 6 by way of receiving-systemcircuits 33 a, 33 b and 33 c. These configuration enable echo signalsdetected by the whole-body coils 7 a, 7 b and 7 c to be received andprocessed independently of each other. The transmitter 8T is coupledwith the whole-body coils 7 a, 7 b and 7 c via the duplexer 31, so thattransmission can be done.

Alternatively, the multiple whole-body coils can be made into aconfiguration shown in FIGS. 13A and 13B. A practical configuration issuch that four-channel whole-body coils 34 a, 34 b, 34 c and 34 d aredisposed around a bobbin, thus the coils being multiple. In thisconfiguration, if the sensitivity distributions are symmetric, somecases are brought to impossible reconstruction. The coils are madeasymmetric in disposal in the lateral and longitudinal directions.

Referring to FIG. 14, another embodiment of the present invention willnow be described. This embodiment relates to a system in which fastimaging that involves couch movement is performed using a singlewhole-body coil.

The fast imaging described by the foregoing paper requires, as theprinciple, N-piece coils (coil elements) to reduce the number ofencoding steps to 1/N. As a result, a magnetic resonance imaging systemthat comprises a single whole-body coil, as seen in conventionalsystems, has a problem that the fast imaging cannot be done.

The present embodiment overcomes this difficulty. Even if only a singlewhole-body is incorporated in the system, the sensitivity distributionof a whole-body coil can be changed relatively by moving the couch. Thisis equivalent to the situation that the same region of an object isscanned with different sensitivity distributions of coils. Based on thisfact, the couch movement is unitized in the present embodiment, so thatthe fast imaging on the principle of reducing the number of encodingsteps is conducted.

For example, at first, the number of encoding steps necessary forreconstructing a single image is reduced into half at a certain couchposition, then scans are performed. Those scans include acoil-sensitivity-distribution-measuring scan and an imaging scan. Thisprovides a single image of a given region to be imaged at the firstcouch position. The couch tabletop is then moved by a predetermineddistance (for example, a distance that corresponds to half of a lengthof the whole-body coil in the Z-axis direction). Like the above, inorder to obtain a further image, the same region of the object issubject to scans with half of the number of encoding steps. Those scanscarried out at the second couch position include a furthercoil-sensitivity-distribution-measuring scan and a further imaging scan.Namely, moving the couch tabletop by the given distance is equivalent tothe situation that there are virtually two whole-body coils (refer toFIG. 14). The whole-body coil is apparently two in number to a givenregion to be imaged of the object, even if only a single whole-body coilis adopted.

Each of the images obtained by the two times of scan causes an aliasingphenomenon therein. However, since being equivalent to two imagesobtained with whole-body coils of different sensitivity distributions, asingle original image is reconstructed, from which the aliasing isremoved by calculation on the technique that the foregoing paperprovides.

In particular, the imaging according to the present embodiment ispreferable to an ultra short-axis type of magnetic resonance imagingsystem. This system, which comprises a single whole-body coil, requiresseveral times of scanning to obtain an image of an ordinal region to beimaged in the Z-axis direction, because the axial length in the Z-axisdirection is shorter. For example, the axial length is about 100 cm andits region to be imaged is some 25 cm. The imaging according to thepresent embodiment can be applied to this system. For instance, theread-out direction is assigned to the Z-axis direction, an FOV in theZ-axis direction is doubled (which is for example the same as the FOV ofa region to be imaged by the conventional long-axis type of magneticresonance imaging system), the number of encoding steps is reduced intohalf, and the couch is moved, each time of scan (in total, two times ofscans), by a distance corresponding to one region imaged by theconventional magnetic resonance imaging system. This makes it possiblethat sagital and coronal imaging, which was difficult for the ultrashort-axis type of magnetic resonance imaging system, to be performed inthe same time as that for the ordinal MRI.

In addition, the couch movement and image processing for the foregoingsingle whole-body coil may be applied to the foregoing configuration ofFIG. 12 in which the multiple RF coils are used. Further, such couchmovement and image processing are also applicable to the configurationof the multiple RF coils of positions are fixed as shown in FIG. 11.

Referring to FIGS. 15 to 18, another embodiment of the present inventionwill now be described. Like the seventh embodiment, this embodiment ischaracterized by fast imaging based on changes in sensitivitydistributions of one or more reception coils when a couch is moved.

Particularly one preferred embodiment of the fast imaging is fastcontrast MR angiography (MRA) carried out under injection of an MRcontrast agent in order to obtain images and/or measurement informationin which its contrast effect is reflected.

The present embodiment will be described about a configuration in whichthe foregoing fast imaging is performed as the fast contrast MRangiography, but the configuration is not limited to such fast contrastMRA.

FIG. 15 shows a schematic form of a magnetic resonance imaging systemcapable of performing the fast contrast MRA. This system includes, asshown in FIG. 15, a whole-body (WB) coil 7T serving as a transmissioncoil and a single reception coil 7R, which are fixedly incorporated inthe bore of a magnet 1. The reception coil 7R constitutes the coilmember of the present invention. The reception coil 7R is connected to areceiving-system circuit 87 via a preamplifier 84, so that predeterminedreception processing is done. This reception processing produces echodata, which are then sent to a host computer 6.

An MR contrast agent can be injected by a contrast agent injector 19into an object P to be imaged lied on the tabletop 14T of a couch.

The remaining hardware configurations in this system are identical tothose in FIG. 3, excepting that the receiving-system circuit is one innumber.

Normally, in performing the contrast MRA, imaging is started before theinjection of a contrast agent. And the same region to be imaged isrepeatedly scanned to acquire echo data in sequence. The echo data aresubjected to conversion to image data, which are used for observation ofchanges in intensity resulting from arrival of the contrast agent. Amore precise understanding of temporal changes in intensity requiresthat imaging be done with higher temporal resolution.

Because only one reception coil 7R is used in this embodiment, the fastcontrast MRA will be performed with the couch table 14T moved, like theforegoing seventh embodiment.

Practically, as shown in FIGS. 16A and 16B, the tabletop 14T is moved totwo different positions P1 and P2 of which coil sensitivitydistributions differ from each other. The first tabletop position P1 isdetermined as shown in FIG. 16A. That is, this position P1 is set suchthat a region to be imaged Rima (hatching portion) of an object P iscomparatively shifted from the center of a sensitivity distributionregion Dsens of the reception coil 7R, with the region Rima crossing aboundary of the sensitivity distribution region Rsens. In addition, thesecond tabletop position P2 is obtained by moving the tabletop 14T by apredetermined distance along the Z-axis direction (i.e., thelongitudinal direction of the couch), as shown in FIG. 16B. The secondtabletop position P2 is set such that the region to be imaged Rima ofthe object P is located within the sensitivity distribution region Dsensof the reception coil 7R on condition that both of the regions' centersagree with each other. Thus, the movement of the tabletop 14T will causethe same imaged region Rima to be scanned at two positions whose coilsensitivity distributions differ from each other.

In this fast contrast MRA, preparation scans are performed at the firsttabletop position P1 in advance. The preparation scans include a firstsensitivity distribution measuring scan for measuring the sensitivitydistribution of the reception coil 7R and one time of imaging scan forproducing an MRA image on the sub-encoding method requiring unfoldingprocessing. A second sensitivity distribution measuring scan will bedescribed later. At the first tabletop position P1, its coil sensitivitydistribution Dsens is different from that at the second tabletopposition P2 later described, by amounts that correspond to a shiftedlength of the imaged region Rima from the center of its sensitivitydistribution region Dsens. But degrees of spatial resolution at both ofthe poisons are the same.

The first sensitivity distribution measuring scan is performed one timefor shortening the scan time and employs a faster pulse sequence. Forinstance, when a sequence used by the imaging scan in this contrast MRAis a three-dimensional FFE scan and its scan matrix size is“128×256×32,” a matrix size used by the sensitivity measuring scan isset to, for example, “16×32×4.” The imaging scan uses a desired pulsesequence for contrast MRA in which its encoding steps are reduced intohalf.

By contrast, at the second tabletop position P2, the main scans areperformed. This main scans include a second sensitivity distributionmeasuring scan performed in the similar way to the above and a pluralityof times of imaging scans for respectively producing an MRA image on thesub-encoding method requiring unfolding processing. The secondsensitivity distribution measuring scan is performed one time with amatrix size of lower spatial resolution, such as “16×32×4” inconsideration of a shortened scan time and a scanning purpose, like thefirst one. Each imaging scan is performed using a desiredthree-dimensional pulse sequence for contrast MRA of which encode(phase-encode) steps are reduced into half. By reducing the number ofencode steps in this way, temporal resolution is improved twice.

FIG. 17 shows the processing for the fast contrast MRA instructed by thehost computer 6.

First, the host computer sends a command to the couch driver 14D to movethe tabletop 14T of the couch to the first tabletop position P1 (referto FIG. 16A), then determines if the preparation scans should beexecuted or not (Steps S21 and S22 in FIG. 17).

If determined that the system is ready for the preparation scans, thehost computer 6 instructs the necessary units to perform the firstsensitivity distribution measuring scan, thus echo data being acquiredand stored (Step S23). Then, the imaging scan is instructed to beperformed one time, thus echo data being acquired and stored (Step S24).

The host computer 6 then sends a command to the couch driver 14D to movethe tabletop 14T to the second tabletop position P2 (refer to FIG. 16B),before the determination if or not the main scans should be performed ismade (Steps S25 and S26).

If it is determined that the system is ready for the main scans, thehost computer 6 instructs the necessary units to perform the secondsensitivity distribution measuring scan (Step S27). Thus echo data forsensitivity distribution measurement are acquired and stored into aninner memory of the host computer. The host computer 6 further sends tothe contrast agent injector 19 a command for the injection of a contrastagent (Step S28). In response to this command, the injector 19 starts toinject the contrast agent into the object P after waiting for a certaininterval of time.

The host computer 6 waits for the interval of time predetermined to bein synchronism with the start of injection of the contrast agent, theninstructs the necessary units to perform the imaging scans (Steps S29and S30). Thus echo data on each imaging scan are acquired and storedinto the inner memory of the host computer 6. The imaging scan at thesecond tabletop position P2 is repeated a plurality of times, forexample, to meet a purpose for measurement of an intensity enhancementeffect of the contrast agent.

On completion of a series of movements of the tabletop 14T and scans,the host computer 6 performs a three-dimensional Fourier transform withthe sets of echo data acquired by the coil sensitivity distributionmeasuring scans and the imaging scans, respectively. Images are thusreconstructed (Step S32).

In each of the three-dimensional images for contrast MRA acquired at thesecond tabletop position P2, aliasing is caused along their encodedirections (phase-encode direction and slice encode direction).Therefore, the host computer 6 uses the previously acquired data) tounfold each set of a three-dimensional image data acquired through theimaging scans at the second table position P2 (Steps S33 and S34). Thoseprevious data include coil sensitivity distribution measurements (imagedata) and image data from the one time of imaging scan, both areacquired at the first tabletop position P1, and coil sensitivitydistribution measurements (image data) acquired at the second tabletopposition P1 for the contrast MRA. The unfolding uses the foregoing knowntechnique.

This unfolding process removes folded data (aliasing) from thethree-dimensional image data at the second tabletop position P2. Thoseimage data are then subject to various kinds of measurement and display(Step S35).

FIG. 18 pictorially shows two timing flows. One flow shows each scan,the couch movements, and the injection of the contrast agent, while theother shows the relationships of usage of data for the unfolding.

In this way, for the single reception coil, the movement of the couch(tabletop) allows a region to be imaged of an object to be moved to twopositions whose sensitivity distributions for the reception coil aredifferent from each other. Therefore, the fast imaging with less encodestep numbers can be performed as an equivalent way to the foregoingtechnique using a plurality of reception coils. Additionally, at one ofthe two positions, a contrast agent can be injected to scan a region tobe imaged. And removing folded data by an unfolding process lead to fastcontrast MRA with higher temporal resolution.

The imaging has been described with respect to an MRA in the aboveembodiment, but the imaging using a single reception coil can be carriedout with no injection of a contrast agent. In such a case, the fastimaging may be performed on the sub-encoding method, although thereception coil is one in number. In other words, like the above eighthembodiment, a region to be imaged may be moved relatively to the onereception coil, so that a plurality of sets of echo data whosesensitivity to the reception RF coil is changed set by set are obtained.This is virtually equivalent to a configuration in which two receptioncoils are used.

Although the eighth embodiment has used one reception coil, thisembodiment is able to adopt two or more reception coils. In cases thenumber of reception coils is increased, the speed of imaging can bedoubled under a limitation that the same region can be imaged.

Further, a whole-body coil for transmission/reception can be used.

In the above eighth embodiment and its modifications, the scans areperformed at the two positions by changing relative positionalrelationships between the couch and the magnet, such as moving thetabletop. Instead, the scans can be done a large number of times morethan two times. For example, the scans are done as the position ischanged N (>3) times, the imaging can be speed up to N times, as long asthe plurality of scan positions are separated from each other so as tosatisfy a condition for the sub-encoding method. Namely, the conditionis that the linearly independence should be maintained among the coilsensitivity distributions.

Further, each embodiment is able to provide an axial image with highspeeds. To realized this, coils are disposed, as reception coil groups,at upper/lower and right/left side positions of an object so that thosecoils are faced to each other and the imaging on the sub-encoding methodis performed using the sensitivity distribution of each coil.

Although the description above contains many specificities, these shouldnot be construed as limiting the scope of the invention but as merelyproviding illustrations of some of the presently preferred embodimentsof this invention. Thus an originally skill in the art can furtherprovide a wide variety of modifications and changes without departingfrom the gist of this invention, based on the description in theappended claims.

As described above, in the foregoing magnetic resonance imaging systemand MR imaging method, in cases multiple RF coils are used as areception coil to perform imaging based on the moving bed technique, itis unnecessary for an operator to manually switch over coils with thecouch moved. Thus the operation can be saved, the imaging can be speededup, and the couch can be moved precisely.

Further, in cases multiple RF coils are used as a reception coil toperform imaging based on the moving bed technique, fast imaging thatimproves temporal resolution can be performed with ease, providing an MRimage of a higher depiction performance.

Even when only one reception coil is disposed, the moving bed techniquecan be adopted for fast imaging. The moving bed technique can beutilized for fast contrast MR angiography (MRA). Thus the moving bedtechnique can have more generalized in usage.

1. A magnetic resonance imaging system comprising: a magnet configuredto generate a static magnetic field containing a uniform region whosemagnetic intensity is uniform; a couch movable in a predetermineddirection passing through the static magnetic field, an object to beimaged disposed on the couch; a reception RF coil; a sensitivitydistribution acquiring unit configured to perform a measuring scan withthe reception RF coil to acquire sensitivity distributions of thereception RF coil at different positions of the couch with respect tothe reception RE coil; a scanning unit configured to perform an imagingscan under a sub-encoding method at an imaging region of the object bytransmitting an RF pulse to the object; a reception unit configured toreceive through the reception RF coil an echo signal emanated from theimaging scanned region of the object in response to scanning and toprocess the echo signal to obtain echo data; an image producing unitconfigured to produce an MR image from echo data obtained in response tothe imaging scan; and an unfolding unit configured to unfold the MRimage under the sub-encoding method using echo data of the sensitivitydistributions.
 2. The magnetic resonance imaging system of claim 1,wherein the reception RF coil comprises a plurality of RF coilsconstituting a multiple RF coil, the predetermined direction is alongitudinal direction of the couch, and positions of the plurality ofRF coils in the predetermined direction are fixed to a position of thecouch.
 3. The magnetic resonance imaging system of claim 2, furthercomprising: a position changing unit configured to automatically changeboth of a relative position between the couch and the magnet and arelative position between the reception RF coil and the couch in thepredetermined direction, wherein the position changing unit isconfigured to change the position so that a center position of each ofthe plurality of RF coils in the longitudinal direction corresponds tothe uniform region of the static magnetic field.
 4. The magneticresonance imaging system of claim 3, wherein the reception unitcomprises selection means for automatically selecting, from the echosignals received by each of the plurality of RF coils, the echo signalreceived by a certain RF coil located at a center of the uniform regionin the longitudinal direction, an output of the selection meansconfigured to be routed to the image producing unit.
 5. The magneticresonance imaging system of claim 4, wherein the selection meanscomprises: signal level detecting means for detecting a level of theecho signal received by each of the plurality of RF coils; and signalselecting means for automatically selecting the echo signal received bythe RF coil located at the center of the uniform region in thelongitudinal direction based on changes in the level of the echo signaldetected by the signal level detecting means.
 6. The magnetic resonanceimaging system of claim 3, further comprising: an identificationgenerating unit configured to generate an identification number of eachRF coil, the identification generating unit being disposed at each ofthe plurality of RF coils, a size memorizing unit configured to memorizea size of each of the plurality of RF coils in the longitudinaldirection, the size corresponding to the identification number of eachRF coil generated by the identification generating unit, a disposaldetecting unit configured to identify each signal line of the pluralityof RF coils so as to detect a disposal state of the plurality of RFcoils in the longitudinal direction, and a determination unit configuredto determine the size based on detection information about the coildisposal state detected by the disposal detecting unit and to providethe size to the size memorizing unit, wherein the position changing unitincludes means for moving the position of the couch based on the sizedetermined by the determination unit, and the reception unit comprisingselection means for automatically selecting, from the echo signalreceived by each of the plurality of RF coils, an echo signal receivedby the RF coil located at a center of the uniform region in thelongitudinal direction on the basis of the size determined by thedetermination unit and the coil disposal state detected by the disposaldetecting unit, the detected echo signal configured to be provided tothe image producing unit.
 7. The magnetic resonance imaging system ofclaim 2, further comprising: a position changing unit configured toautomatically change both of a relative position between the couch andthe magnet and a relative position between the reception RF coil and thecouch in the predetermined direction, wherein: the position changingunit comprises means for changing the position so that, of the pluralityof RF coils constituting the multiple RF coil, an overlapped region ofsensitivity distribution regions of any two RF coils which aremutually-adjoining corresponds with the uniform region of the staticmagnetic field in the longitudinal direction and the position changingunit configured to move the couch step by step by a distancecorresponding to each RF coil in the longitudinal direction, and theimage producing unit comprises means for performing unfolding processingon a set of echo data obtained by the reception unit at every positionof the couch changed by the position changing unit on the basis of thesensitivity distributions of the plurality of RF coils.
 8. The magneticresonance imaging system of claim 1, wherein the reception RF coilcomprises a plurality of RF coils defining a multiple RF coil, each ofthe plurality of RF coils comprising defining the multiple RF coil beingan array type of RF coil having a plurality of coil elements.
 9. Themagnetic resonance imaging system of claim 1, wherein the reception RFcoil comprises a plurality of RF coils defining a multiple RF coil, eachof the plurality of coil members defining the multiple RF coil is awhole-body coil.
 10. The magnetic resonance imaging system of claim 1,wherein the reception RF coil comprises a multiple RF coil fixed to oneof the object or the couch.
 11. The magnetic resonance imaging system ofclaim 1, wherein the reception RF coil is one in number.
 12. A magneticresonance imaging system comprising: static magnetic field generatingmeans for generating a static magnetic field containing a uniform regionwhose magnetic intensity is uniform; a couch movable in a predetermineddirection passing through the static magnetic field, an object to beimaged disposed on the couch; a reception RF coil; position changingmeans for changing both of a relative position between the couch and thestatic magnetic field generating means and a relative position betweenthe reception RF coil and the couch in the predetermined direction; asensitivity distribution acquiring means for performing a measuring scanwith the reception RF coil to acquire sensitivity distributions of thereception RF coil at different positions of the couch with respect tothe reception RF coil; scanning means for performing an imaging scanunder a sub-encoding method at an imaging region of the object bytransmitting an RF pulse to the object; reception means for receivingthrough the reception RF coil an echo signal emanated from the imagingscanned region of the object in response to scanning;reception-processing means for processing the echo signal received bythe reception means into echo data; image producing means for producingMR images from echo data obtained in response to the imaging scan; andunfolding performing means for unfolding the MR images under thesub-encoding method, using echo data of the sensitivity distributions.13. A magnetic resonance imaging system comprising: a magnet configuredto generate a static magnetic field containing a uniform region whosemagnetic intensity is uniform; a couch movable in a predetermineddirection passing through the static magnetic field, an object to beimaged disposed on the couch; a reception RF coil; a position changingunit configured to change both of a relative position between the couchand the magnet and a relative position between the reception RF coil andthe couch in the predetermined direction; a sensitivity distributionacquiring unit configured to perform a measuring scan with the receptionRF coil to acquire sensitivity distributions of the reception RF coil atdifferent positions of the couch with respect to the reception RF coil;a scanning unit configured to perform an imaging scan under asub-encoding method at an imaging region of the object by transmittingan RF pulse to the object; a reception unit configured to receivethrough the reception RF coil an echo signal emanated from the imagingscanned region of the object in response to the scanning and to processthe echo signal to obtain echo data; an image producing unit configuredto produce MR signals from echo data obtained in response to the imagingscan; and an unfolding unit configured to unfold the MR images under asub-encoding method using echo data of the sensitivity distributions.14. The magnetic resonance imaging system of claim 13, wherein thereception RF coil is one in number.
 15. The magnetic resonance imagingsystem of claim 14, wherein the reception RF coil is a whole-body coilconfigured to be used for both the transmission of the RF pulse and areception of the echo data.
 16. The magnetic resonance imaging system ofclaim 15, wherein the position changing unit is configured to move thecouch every half of a length of the reception RF coil in thepredetermined direction.
 17. The magnetic resonance imaging system ofclaim 13, wherein the position changing unit is configured to move thecouch to a first couch position and a second couch position, a region tobe imaged of the object located at the first couch position configuredto be shifted in part from a sensitivity distribution region of thereception RF coil; and the region located at the second couch positionwith the region being contained entirely in the sensitivity distributionregion of the reception RF coil, the system further including aninstruction unit configured to instruct a contrast agent to be injectedinto the object when the couch is located at the second position. 18.The magnetic resonance imaging system of claim 17, wherein: themeasuring scan is comprised of both a first sensitivity-distributionmeasuring scan for measuring a sensitivity distribution of the receptionRF coil carried out when the couch is located at the first couchposition and a second sensitivity-distribution measuring scan formeasuring a sensitivity distribution of the reception RF coil carriedout when the couch is located at the second couch position; the imagingscan comprising both a first imaging scan for obtaining the MR images ofthe region carried out when the couch is located at the first couchposition and a plurality of second imaging scans for obtaining the MRimages of the region carried out when the couch is located at the secondcouch position; the image producing unit is configured to reconstructecho data obtained by both of the first and second imaging scans intoimage data; and the unfolding unit is configured to unfold the imagedata obtained through each of the second imaging scans using both echodata obtained through the first and second sensitivity-distributionmeasuring scans and the image data obtained through the first imagingscan.
 19. The magnetic resonance imaging system of claim 17, wherein thereception RF coil is one in number.
 20. A magnetic resonance imagingsystem comprising: a magnet configured to generate a static magneticfield containing a uniform region whose magnetic intensity is uniform; acouch movable in a longitudinal direction passing though the staticmagnetic field, an object to be imaged disposed on the couch; areception multiple RF coil including a plurality of coil memberspositioned around the object; a position changing unit configured tochange a relative position of the couch with respect to the magnet inthe longitudinal direction when the object is being imaged; asensitivity distribution acquiring unit configured to perform ameasuring scan with the reception multiple RF coil to acquiresensitivity distributions of the reception multiple RF coil at differentpositions of the couch with respect to the reception multiple RF coil,the different positions being given by the position changing unit; ascanning unit configured to perform an imaging scan under a sub-encodingmethod of the object by applying a train of pulses to the object; areception unit configured to receive through the reception multiple RFcoil an echo signal in response to the train of pulses applied by thescanning unit; a reception-processing unit comprising a selectionelement configured to automatically select, from echo signals receivedindividually by the plurality of coil members, an echo signal receivedby a coil member located at a center of the uniform region in thelongitudinal direction and a processing element configured to processthe selected echo signal into echo data; an image producing unitconfigured to produce an MR image based on the echo data processed bythe reception-processing unit; and an unfolding unit configured tounfold the MR image under a sub-encoding method using echo data of thesensitivity distributions.
 21. A magnetic resonance imaging systemcomprising: a magnet configured to generate a static magnetic fieldcontaining a uniform region whose magnetic intensity is uniform; a couchmovable in a longitudinal direction passing through the static magneticfield, an object to be imaged disposed on the couch; a receptionmultiple RF coil including a plurality of coil members positioned aroundthe object; a coil position setting unit configured to set a position ofeach of the coil members in the longitudinal direction; a positionchanging unit configured to change a relative position of the couch withrespect to the magnet in the longitudinal direction when the object isbeing imaged, on the basis of the position of each of the coil membersset by the coil position setting unit; a sensitivity distributionacquiring unit configured to perform a measuring scan with the receptionmultiple RF coil to acquire sensitivity distributions of the receptionmultiple RF coil at different positions of the couch with respect to thereception multiple RF coil, the different positions being given by theposition changing unit; a scanning unit configured to perform an imagingscan under a sub-encoding method of the object by applying a train ofpulses to the object; a reception unit configured to receive through thereception multiple RF coil an echo signal in response to the train ofpulses applied by the scanning unit; an image producing unit configuredto produce an MR image based on the echo signal received by thereception unit; and an unfolding unit configured to unfold the MR imageunder a sub-encoding method using echo data of the sensitivitydistributions.
 22. The magnetic resonance imaging system of claim 21,wherein the coil position setting unit is configured to set the positionof each coil member based on information showing a central position ofeach coil member in the longitudinal direction, a size of each coilmember in the longitudinal direction, and a current position in thelongitudinal direction.
 23. The magnetic resonance imaging system ofclaim 22, wherein the information showing the central position of eachcoil member in the longitudinal direction is provided by a markerpreviously put at the central position of each coil member.
 24. Amagnetic resonance imaging system comprising: a magnet configured togenerate a static magnetic field containing a uniform region whosemagnetic intensity is uniform; a couch movable in a longitudinaldirection passing through the static magnetic field, an object to beimaged disposed on the couch; a reception multiple RF coil including aplurality of coil members positioned around the object, each coil memberhaving a pickup coil; a coil position setting unit configured to set aposition of each of the coil members in the longitudinal direction byusing an output signal from each of the pickup coils; a positionchanging unit configured to change a relative position of the couch withrespect to the magnet in the longitudinal direction when the object isbeing imaged, on the basis of the position of each of the coil membersset by the coil position setting unit; a sensitivity distributionacquiring unit configured to perform a measuring scan with the receptionmultiple RF coil to acquire sensitivity distributions of the receptionmultiple RF coil at different positions of the couch with respect to thereception multiple RF coil, the different positions being given by theposition changing unit; a scanning unit configured to perform an imagingscan under a sub-encoding method of the object by applying a train ofpulses to the object; a reception unit configured to receive through thereception multiple RF coil an echo signal in response to the train ofpulses applied by the scanning unit; an image producing unit configuredto produce an MR image based on the echo signal received by thereception unit; and an unfolding unit configured to unfold the MR imageunder a sub-encoding method using echo data of the sensitivitydistributions.